Being excellent in the gas barrier property and chemical resistance in addition to mechanical characteristic and impact resistance, polybutylene terephthalate (hereinafter, referred to as PBT) has conventionally used as an engineering plastic and particularly as a useful material owing to good productivity attributed to the crystallization speed. However, PBT has high crystallization speed and its biaxial stretch has been considered to be difficult. That is because crystallization occurs due to stretch in stretching process and thus stretching becomes difficult.
There is a technique known for producing a biaxially stretched PBT film by stretching in the TD direction at a stretch ratio of 3.5 times or lower and successively in the MD direction at a deformation speed of 100000%/min to produce an evenly stretched film with no thickness unevenness (see, for example, Patent Document 1). However, as being indicated from the results of Examples, such a conventional technique has a problem that a film produced by the technique has low elongation and is inferior in transparency and dimensional stability because of high deformation speed only in the MD direction and therefore fails to be in good balance between the MD direction and the TD direction (see, for example, Patent Document 1).
Regarding an un-stretched PBT film, there is a technique known for keeping piercing displacement within a specified range to provide excellent processing suitability for uses to carry out drawing formation, such as an exterior material for lithium ion batteries (see, for example, Patent Document 2).
However, such a conventional technique has a problem that stretch of PBT is weak since PBT is not stretched and the intrinsic characteristics of PBT are not sufficiently extracted in terms of mechanical characteristic and impact resistance.
Accordingly, in order to advantageously utilize the intrinsic characteristics of PBT, investigations for the purpose of enhancing plane orientation by biaxial stretch and improving mechanical characteristic and impact resistance have been made for past 40 years or more. Some of past investigations on a PBT film will be examined.
For example, there is a technique known for producing a film with slight anisotropy and excellent in mechanical properties and dimensional stability by producing a PBT film with rupture strength of specified values or higher in 4 directions by employing a tubular and simultaneous biaxial stretch method (see, for example, Patent Document 3).
However, such a conventional technique has a problem that the thickness precision is inferior attributed to the production method and plane orientation coefficient is not high and therefore, piercing strength is low.
Further, there is a technique known for providing high rigidity and excellent dimensional stability and formability at a high temperature by alternately and solely layering two kind resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) besides PBT in a large number of layers (see, for example, Patent Document 4).
However, such a technique has a problem that layers of resins such as PET and PEN other than PBT are layered and Tg of PET or PEN is higher than Tg of PBT, and subsequently, elongation of PBT is carried out at high temperature and therefore the elongation of PBT is at such a high temperature as to fail to extract the intrinsic characteristics of the PBT film and additionally a problem that resin composition of the film contains two kind resins and therefore it is difficult to reuse trimming dust generated at the time of film formation by adding again to starting materials.
As described above, conventional biaxially stretched polybutylene terephthalate films have no capability sufficient for uses as wrapping materials and exterior materials for lithium batteries.